1. Field of the Invention
This invention relates to a low-density polyethylene composition useful for coating polar substrates. This invention particularly relates to a low-density polyethylene composition useful for coating metal foils.
2. Description of the Related Art
Low density polyethylene (LDPE) can be used in packaging applications and/or other applications as a coating on polar substrates such as aluminum foil, paper, primed films, and metallized films, for example. LDPE can be useful as either a sealant, as an adhesive, or both, in these applications.
Other methods of coating LDPE on a polar surface are known, but typically LDPE can either be extrusion-coated or laminated onto a polar substrate. In such a process, the polymer is melted and coated as a curtain—that is as a thin, wide band of polymer melt carefully poured onto the substrate—as the substrate is pulled across a cold chill roll at high speed. The LDPE freezes on the cold surface of the substrate. To promote adhesion in this high-speed process, the polymer curtain can be oxidized by application of high temperature with long exposure to air or ozone, flame, corona, or other treatments. The results are inconsistent and the strength of the adhesion is not reliable, and the resulting bond is typically weak. Other problems encountered while using this process are that the coated substrate can have poor organoleptic properties, there is off-gassing, smoke generation, polymer degradation, and other processing problems.
Acid copolymer resins (ACR's), such as Nucrel®, and ionomers such as Surlyn®, available from E. I. du Pont de Nemours and Company can be used in place of LDPE to bond to polar substrates in extrusion coating or lamination processes. Resins such as these can bond well to foils in high line-speed processes having short exposure times and lower melt temperatures. These resins can include from 6 to 12 wt. % methacrylic acid or acrylic acid, and therefore can be more expensive than LDPE. Consequently, acid copolymers and ionomers are more typically used in the more demanding applications.
Some applications are moderately demanding, and can utilize acid copolymers having a lower acid content. For example, resin grades having only 4 wt % methacrylic acid, and only 3 wt % acrylic acid are available commercially for use in applications wherein only moderate performance is required of the adhesive. However, these resins can nevertheless be too expensive for most applications.
A resin with grafted anhydride functionality can be blended with LDPE and can improve adhesion to foil, but anhydride grafted resins do not bond to polar substrates well in high speed extrusion coating processes. Rubber and/or other elastomeric materials are typically added to resins having grafted anhydride functionality to improve adhesive properties. Polyolefins having acrylic acid grafted functionality of 6 wt % can be purchased commercially. These polymers cannot be processed at temperatures typical in an extrusion coating process, typically from 260° C. to 320° C. Blends of high acid copolymers or ionomers, having acid content of from 6% to 12%, with LDPE have been considered for improving adhesion of LDPE to foil because (1) high acid resin grades are normally used in extrusion coating, and (2) resins having high-acid content normally give better adhesion to foil than low acid resins. Surprisingly, however, in some cases blending high-acid acid copolymer resins with LDPE does not enhance the adhesion of LDPE to foil. More surprising in view of this is that blends of low acid copolymer resins with LDPE have significantly improved adhesion to polar substrates over LDPE.